Hang around any mom with a young child and eventually she’ll break out her baby voice. You know the one — the pitch of her voice goes up, her words are simple and exaggerated. It’s sometimes referred to as motherese, but researchers call it infant-directed speech. Whatever you want to call it, it’s pretty vital to little ones’ development. Says Elise Piazza, a neuroscientist at Princeton University, it “helps babies to segment this huge stream of words into the building blocks of language.”

Researchers have known for a while that mothers change these more basic aspects of their speech, such as pitch (how high or low a tone sounds) and word choice, when they’re talking to their infants. But now, Piazza and her team have pinpointed a more subtle change, to something called timbre.

“We use timbre descriptors all the time,” she explains. “For instance, the nasality of Gilbert Gottfried’s voice or the velvety tone of Pavarotti — we can discriminate these two speakers, even though they might be speaking in the same pitches. We also use timbre descriptors when talking about music. So we might have the reedy woodwinds or the buzzy brass. These have nothing to do with pitch or rhythm; they’re timbre descriptors.”

To find out if moms adjust this vocal characteristic, Piazza and her colleagues recorded two scenarios: 12 English-speaking women talking to their 7- to 12- month-old children and then talking to another adult. Next, using audio clips from these sessions, they calculated the mathematical profile of each woman’s vocal spectrum — essentially, they found the fingerprint of each mom’s timbre.

And when they compared the timbre fingerprints from when moms were talking to their babies with those from when they were just talking to adults, the team found a distinct difference. That difference is apparent even in non-english speaking mothers, too. After seeing the results from the first group, the researchers tested 12 non-native English speakers in the same scenarios. Except, this time, the moms cooed at their babes in their native tongue. The results were the same.

Yes, the findings add to an already huge pile of literature on motherese. But knowing moms are changing even complex vocal characteristics like timbre could help experts fine-tune things like speech recognition software. “It could tease apart how much speech is really being directed at [babies], consciously targeting their own needs, versus overheard conversation,” Piazza says. “And to be able to do that instantaneously across many languages could get into some interesting cross cultural questions in a variety of environments around the world.”